Laser beam levelling instruments of the aforementioned type are used for fixing a horizontal plane and are in particular used for building purposes. The laser beam passing out of the instrument in a generally invisible manner is caused to rotate horizontally by a rotary head. The rotary laser beam can then be received by a receiver at a random point around the levelling instrument and rendered visible. With the aid of a stadia rod adjustably fitted to the receiver it is then possible to carry out many different measuring and control functions. Laser beam levelling instruments are used, for example, in excavating soil and levelling foundation trenches, in checking concrete foundations and floors, in aligning forms, when levelling tracks for high superstructure cranes, when applying height markings and for many other purposes.
Prior to the start of the work, a laser beam levelling instrument must itself be horizontal on a frame or support and for this purpose spirit levels are often positioned externally on the instrument. To obviate a time-consuming fine setting on each occasion, it is advantageous if the actual instrument has an automatic levelling function in a maximum broad range. Automatic levelling of the instrument is also very advantageous because the instrument can easily be brought out of a horizontal position on a building site due to vibrations or shocks during surveying work. However, the results would be falsified by an unnoticed lateral tilt of the instrument.
In hitherto known instruments, automatic levelling has generally been achieved by a gimbal mount, so that these instruments are very large and cumbersome, particularly if servomotors are also used for assisting the automatic levelling. Attempts have also been made to suspend the rotary head with reflection means on wires. Admittedly, this reduces the size of the instrument but the actual automatic levelling can only take place in a very small range.